This invention relates to a system for automatically monitoring, maintaining and adjusting the concentration of a material carried in a fluid carrier or medium and is particularly useful for monitoring and adjusting toner material suspended in a fluid carrier in the development system of an electrographic printer/plotter.
In the past, a system for monitoring and maintaining the material concentration in a fluid carrier or medium, in particular toner concentrate in a fluid carrier, has been normally carried out by examining the transmissivity of the fluid carrier as it passes into or through a fluid carrier chamber or as it passes through a flow cell. Examples of such systems are disclosed in U.S. Pat. Nos. 3,650,196; 3,699,992; 3,712,203, 3,926,145; 3,992,109; 4,077,724; 4,119,989; 4,124,301; 4,222,497 and 4,310,238. All of the systems disclosed in these patents include some optical or electrical means to sense, detect or measure the transmissivity of the fluid carrier and determine a change in that transmissivity. In many cases the change in transmissivity is compared to some reference, and when appropriate, additional concentrate material is added to the fluid carrier to replenish the carrier solution.
U.S. Pat. No. 4,222,497, assigned to the assignee herein, relates to a more sophisticated toner concentrate control system compared to most of these previous systems in that measurements of transmissivity of the fluid carrier are more accurately determined by periodically determining the transmissivity of a flow cell when no fluid carrier is in the cell, which measurement is substrate from the measurement of transmissivity of the fluid carrier when present and flowing through the cell. In this manner, the transmissivity of the fluid carrier per se may be accurately determined. In establishing a predetermined standard of measurement by eliminating effects of aging of the illumination source and photodetector for the flow cell or the effects of stained conditions developed on the interior walls of the flow cell over a period of time, an accurate determination of toner concentrate depletion in the fluid carrier can be realized.
The system of U.S. Pat. No. 4,222,497 also provides a means to discontinue pumping of the toner fluid carrier throughout the system in the event that comparison values indicate that apparently no fluid carrier is in the flow cell or the system is being pumped with air bubbles or other highly transmissive material. A signal is generated under such conditions, which is operative to shut off the toner pump. It is not desirable to make transmissivity measurements when air bubbles are passing through the flow cell because they provide a transmissivity measurement of high value indicating a depletion of toner concentrate has occurred requiring replenishment when, in fact, the toner concentrate may not need replenishing.
What is needed is some means by which the system can monitor the transmissivity of toner concentrate and "ignore" the passage of air bubbles or other sudden changes in the monitored toner transmissivity while passing through the system flow cell. In this manner, the system is "smart" in only requiring the addition of toner concentrate to the fluid carrier, when the toner concentrate does actually need replenishing.